A copending and commonly assigned United States patent application of Robert A. Sprague, which was filed Dec. 4, 1984 under Ser. No. 678,199 on a "Distributed Data Storage Architecture for Block Oriented Solid State Optical Memories", discloses a block oriented solid state optical memory in which the bits of each of the optically addressable data blocks are stored in spatially distributed memory sectors. That is a fundamental departure from the solid state optical memories which others are known to have proposed because those memories utilized a "concentrated page storage architecture," whereby each page or block of data is concentrated within a single, optically addressable memory "sector", so that it may be retrieved on demand by selectively illuminating or "addressing" just that one sector. See, for example, U.S. Pat. Nos. 3,676,864, which issued June 29, 1970 on an "Optical Memory Apparatus"; 3,765,749, which issued May 23, 1972 on an "Optical Memory Storage and Retrieval System"; and 3,899,778, which issued Aug. 12, 1975 on "Means Employing a Multiple Lens Array for Reading From a High Density Optical Memory Storage."
In view of the coined descriptors which have been adopted to provide convenient names for the above-described storage architectures, it may be helpful to note that the phrases "block of data" and "page of data" are used herein more or less interchangeably. Some of the prior art refers to the storage of optically addressable "pages," so it has been characterized as suggesting a concentrated page storage architecture. Similar terminology could be used to describe the general organization of the data stored in a memory embodying this invention, but the preferred terminology provides a more generic description which literally applies to all cases, including one wherein each optically addressable data block comprises a plurality of electrically addressable data segments. Since such data segments might be characterized as being separate "pages of data" as that phrase is normally used in the data processing field, alternative terminology has been adopted to prevent any confusion.
One of the principal advantages of a distributed data storage architecture is that it increases the tolerance of block oriented solid state optical memories to localized internal optical defects, which commonly are caused by dust particles and other foreign matter that are inadvertantly or unadvoidably embedded in such memories during their fabrication or use. Gross optical defects are avoided relatively easily by exercising reasonable care during the manufacture and maintenance of such a memory, but it is economically and technically unrealistic to insist upon optical perfection. Accordingly, solid state optical memories are likely to contain relatively small localized internal optical defects which optically mask, in whole or part, some of the bits stored therein.
The practical consequences of such optical defects are more significant for memories having high bit densities, such as miniaturized units and larger, high capacity units, than for those that have more modest bit densities. Indeed, there are existing error correction techniques to adequately correct for isolated bit errors and for short and infrequent burst errors, so the limiting effect of the optical defects is not fully realized until the bit density is increased sufficiently to cause their optical masking effect to produce excessively long and/or frequent burst errors in the data read out from the memory. The bit density at which that occurs is dependent on a number of variables, including the optical quality control standards applied duing the memory manufacturing process, the cleanliness of the environment in which the memory is used, and the sophistication of any error correction procedure that is employed. However, it will be evident that a memory having a concentrated page storage architecture reaches that limit at a much lower bit density than a comparable memory having a distributed data storage architecture.
Due to the technical superiority of a distributed data storage architecture for block oriented solid state memories, the goal now is to more fully realize the potential of the technology. It has been shown that such a memory may include a shadow mask or the like for geometrically projecting images of an array of photoemitters in parallel onto spatially displaced sections of a data mask which, in turn, are optically aligned with respective photosensors. That optically partitons the memory so that it has a plurality of spatially distributed memory sectors and optically subdivides each of the sectors into a plurality of selectively illuminable or "addressable" memory cells. Furthermore, a compatible data storage strategy has been developed for mapping the bits of multi-bit data blocks onto the data mask, so that the bits of each data block occupy the memory cells which are illuminable or "optically addressable" by a respective one of the photoemitters.
As will be appreciated, the maximum permissible bit storage density of a memory of the foregoing type is limited by the resolution with which the photoemitter array is imaged onto the data mask. Indeed, in view of the increased tolerance of such memories to localized internal optical defects, the resolution limit generally determines their maximum permissible bit storage density/memory sector. Some increased imaging resolution might be achieved relatively straightforwardly, such as by substituting more or less conventional imaging lenses for the apertures of the aforementioned shadow mask, but a carefully optimized imaging system is required to fully realize the advantages of the distributed data storage architecture.